Ever been to a dyno? It measures torque and hp at the wheels. Typically all cars dyno in the gear closest to 1:1 which iirc is 5th gear in the m3...

Perhaps a nit, but chassis dynos don't measure torque. They directly measure power thru acceleration of a known mass (with a known rotational inertia) at known speeds.

Proof point: If you don't hook up a sensor to a plug wire to get rpm, you'll still get a power curve, but no torque curve. Getting rpm means you can then calculate the torque curve.

Running the rollers in differing gears will get you similar results, because greater acceleration of the rollers in, say, first gear is offset by that happening at lower speeds - in a proportional way.